1. Field of the Invention
This invention relates to laser markable articles and documents made therefrom, especially security documents.
2. Description of the Related Art
Security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various paper or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body. A principal objective of such security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by local heating of material, while in laser engraving material is removed by laser ablation.
Well known in the field of laser markable security documents is the use of laser markable polymeric supports. Laser marking produces a colour change from white to black in a laser markable support through carbonization of the polymer, usually polycarbonate as disclosed in e.g. EP-A 2181858 (AGFA GEVAERT).
During the past last years, there is an increased interest of using laser markable layers. The advantage of using a laser markable layer coated on a support instead of a laser markable support, is that a support can be used which has better physical properties than the laser markable supports, such as for example a higher flexibility than a polycarbonate support as disclosed in e.g. EP-A 2567825 (AGFA GEVAERT).
Laser markable layers are composed of colour forming compounds (also called “leuco-dyes”) which can change from essentially colourless or pale-coloured to coloured when irradiated with UV light, IR light and/or heated. Different classes of leuco dyes are well known and widely used in conventional pressure-sensitive, photosensitive or thermally-sensitive recording materials (“Chemistry and Applications of Leuco Dyes”, Ramaiah Muthyala, Plenum Press, 1997).
Leuco-dyes for laser-markable layers may be used in combination with acid-generating compounds such as photoacid-generators (PAG). Photoacid generators are for example widely used in conventional photo resist material. For more information about these photoacid generators and photosensitizing dyes, see HINSBERG, W. D., et al. Encyclopedia of Polymer Science and Technology: Lithographic resists. Edited by JOHN WILEY, 2012. p. 13-16 or GREEN, W. Arthur. Industrial Photoinitiators, A technical Guide. Edited by CRC PRESS. NW: Taylor, 2010. p. 83, 150-164. Some examples of classes of photoacid generators and H-donor precursors are iodonium salts, sulfonium salts, ferrocenium salts, sulfonyl oximes, halomethyl triazines, halomethylarylsulfone, α-haloacetophenones, sulfonate esters, t-butyl esters, allyl substituted phenols or t-butyl carbonates.
A disadvantage of a material, for example a colour laser markable laminate including such a colour laser markable layer, may be their limited daylight stability.
The daylight stability of laminates may be improved by the addition of daylight stabilizers (more information on daylight stabilizers can be found in “Plastics Additives Handbook, 5th Edition”, Hans Zweifel, HANSER, 2001).
However, the addition of daylight stabilizers may negatively impact the performance of the laminate, such as giving significantly less colour formation upon laser marking (loss of sensitivity). Such a lower sensitivity requires longer exposure time of the laser.
A further requirement of colour laser markable lamiantes for security cards is an excellent thermal stability. Lamination is usually performed at elevated temperatures in order to achieve a good adhesion between the different layers. Laser markable layers comprising leuco-dyes and acid generators may have a limited thermal stability which results in background stain of the laser markable material after lamination at elevated temperatures.
WO2007063339 (DATALASE) discloses a composition comprising a dye responsive to the presence of hydrogen ions but substantially non-responsive to irradiation or heating, a compound that generates an acid on irradiation or heating, and a binder. The acid-generating compound is preferentially an ester or amide of an aromatic sulphonic acid. In a preferred embodiment, the acid-generating compound is a tosylate of aromatic hydroxides such as phenyl tosylate and hydroquinone ditosylate.
WO2008127919 (HEWLETT PACKARD) discloses an image recording coating comprising a matrix, a radiation absorbing compound, an activator and a colour former wherein the activator optionally comprises a sulfonylurea derivative.
JP2002046358 (FUJI) discloses a thermal printing material containing a specific class of leuco dyes, an IR-absorbent and an acid generator.
JP9302236 (TEIJIN) discloses a laser-markable resin composition which comprises a thermoplastic polymer, a leuco-dye, and an aromatic halogenide or sulfonate ester.